Chronic Hepatitis B Virus (HBV) infection is a major factor in the pathogenesis of hepatocellular carcinoma (HCC). Despite the HBV vaccine, the World Health Organization reports 400 million people are chronically infected with HBV. Current treatments are ineffective, and rates of primary HCC have tripled in the U.S. from 1975-2005. To reduce liver cancer, new and effective therapies are needed, as well as new biomarkers for molecular classification and prognosis of the disease. In this proposal we investigate the role of the mitotic Polo-like kinase1 (Plk1) in the pathogenesis of HBV-mediated liver cancer (HBV-HCC). Pathogenesis of HBV-HCC involves chronic liver inflammation and effects of the weakly oncogenic HBV X protein (pX). pX activates cellular mitogenic pathways, promotes DNA re-replication-induced DNA damage and activates Plk1. In turn, Plk1 mediates checkpoint adaptation, generating partial polyploidy in non- transformed pX-expressing hepatocytes. Significantly, inhibition of Plk1 suppresses pX-mediated transformation. However much remains to be understood about the role of Plk1 in pX-mediated transformation and HBV-HCC. Toward this end, we have identified by a genome-wide siRNA library screen, SUZ12 and ZNF198 as tumor suppressors of pX-mediated transformation. Our results indicate that these proteins are negatively regulated by Plk1. Both in human liver cancer cell lines and tissues from chronic HBV patients with HCC, protein levels of Plk1 are increased, whereas those of SUZ12 and ZNF198 are reduced relative to normal controls. This inverse relationship (high protein levels of Plk1 and reduced levels of SUZ12 & ZNF198) also occurs during HBV replication, suggesting overexpression of Plk1 and down-regulation of SUZ12 & ZNF198 is important both for transformation and HBV replication. SUZ12 and ZNF198 mediate chromatin remodeling and associate with PML nuclear bodies (NBs) that regulate DNA repair, apoptosis and viral replication. We reason down-regulation of SUZ12 & ZNF198 by Plk1 alters hepatocyte gene expression and disrupts the function of PML NBs. Accordingly, our hypothesis is: Plk1 down-regulates SUZ12 and ZNF198, which enhances HBV replication by disrupting PML NBs, and mediates oncogenic transformation by deregulating hepatocyte gene expression. We will investigate in Aim 1, the role of Plk1, SUZ12, and ZNF198 in pX-mediated transformation and HBV replication; in Aim 2, the mechanism by which Plk1 down-regulates ZNF198 and SUZ12. In Aim 3, we will investigate whether protein levels of Plk1, SUZ12, ZNF198, and expression of known SUZ12 target genes are prognostic for disease progression and survival. Impact: High level of viremia in chronic HBV patients is a risk factor for progression to HCC. Inhibition of Plk1 could serve as a therapy strategy to suppress HBV replication, reducing the risk of HCC development. Plk1 inhibitors are in clinical trials for other types of cancer and could serve as therapy for HBV-HCC. Our studies hold promise to reveal novel therapy targets and biomarkers for HBV-HCC.